1. Field of Invention
The present invention relates to a fluid compressor that includes: an upper port and a lower port that are located at upper and lower portions, respectively, of a pump chamber; an upper passage in communication with the inside of the pump chamber via the upper port; and a lower passage in communication with the inside of the pump chamber via the lower port; and a rotor disposed in the pump chamber, and a fuel cell vehicle that is equipped with the fluid compressor.
2. Description of Related Art
A fuel cell system is contemplated that has a fuel cell stack that generates electric power through an electrochemical reaction between fuel gas, such as a gas containing hydrogen, and oxidation gas, such as air. The fuel cell stack is constituted by stacking a plurality of fuel cell sets, each of which has a membrane-electrode assembly (MEA) that is constituted of an anode, an electrolyte membrane, and a cathode; and separators, for example. That is, each unit fuel-cell cell is constituted by placing an anode and a cathode, respectively, on opposite sides of an electrolyte membrane that is constituted of a polymer ion-exchange membrane. The MEA is then placed between two separators. A fuel cell stack that generates a high voltage is constituted by stacking a plurality of individual unit fuel cells and sandwiching the stack between current collecting plates, insulating plates, and end plates.
In such a fuel cell, fuel gas is supplied to the anodes and oxidation gas is supplied to the cathodes. Then, the fuel gas and oxidation gas undergo an electrochemical reaction to generate electricity.
For example, a vehicle may be equipped with the above described fuel cell system to supply electric power generated by the fuel cell to a driving motor that drives wheels. In this case, the fuel cell is used as an electric power source for the driving motor.
In a fuel cell system, a fluid compressor, such as an air compressor, is used to supply oxidation gas to the fuel cell. Hydrogen off-gas, i.e., hydrogen gas that is discharged from the fuel cell and contains unreacted hydrogen gas, may also be mixed with fresh hydrogen gas in a circulation passage. The mixed hydrogen gas is to be supplied to the fuel cell to improve fuel efficiency. In this case, the fluid compressor is a hydrogen pump, and the hydrogen pump is provided in the circulation passage.
Japanese Patent Application Publication No. 2005-180421 (JP-A-2005-180421) describes a hydrogen compressor that has a pump chamber defined by the inner surface of a pump housing and the inner surface of a bearing block; and two dual-lobe rotors disposed in the pump chamber. A delivery port is formed at the center of the bottom of the pump chamber, and a guide surface is formed in an inverted circular truncated cone shape that slopes downward toward the opening edge of the delivery port. One benefit of the described hydrogen compressor is that water that has been drawn into or condensed in the pump chamber flows out through the delivery port and does not remain on the bottom of the pump chamber.
If a fluid compressor such as the hydrogen pump or air compressor is left in an environment with a temperature below zero, water retained in gaps between the two rotors or between the rotors and the housing may freeze. In this case, the adherence of ice may make a smooth restart of the fluid compressor difficult or even impossible. Also, if ice is formed in recesses of the rotors, the ice may be caught between the rotors when the fluid compressor is restarted and make a smooth start of fluid compressor difficult. In addition, there is room for improvement in terms of minimizing of damage to the rotors.
Generally, a fuel cell system has a oxidation gas flow path to supply air to the fuel cell and to discharge air off-gas, i.e., the air after the reaction, from the fuel cell. When water vapor contained in the air condenses in the oxidation gas flow path, the water may flow from the upstream side or flow (back) from the downstream side into the air compressor, which is located upstream of the oxidation gas flow path. Then, if the air compressor is left in a low-temperature environment, ice may form in the pump chamber of the air compressor. In the case of a fuel cell vehicle equipped with a fuel cell system, due to the temperature difference that generally occurs between the inside and outside of the chamber, water tends to condense as described above. Further, water or snow thrown up when the fuel cell vehicle runs in the rain or on the snow may be drawn in by the air compressor, and freeze within the compressor.
In view of the above, a way to prevent entry of an excessive amount water into a fluid compressor is desirable. In contrast, with the hydrogen compressor described in JP-A-2005-180421, ensuring that the water present in the pump chamber flows smoothly toward the delivery port is taken into account, but preventing the entry of water into the pump chamber is not taken into account. It is also contemplated to increase the output of the motor that drives the rotors or to carry out a scavenging operation to direct a fluid such as air therethrough by driving the fluid compressor at a relatively high rotational speed for a certain period of time after the fuel cell system is shut down, in order to make a restart of the fluid compressor possible even if ice has formed therein and to discharge the water in the fluid compressor. Such a structure, however, may waste energy.